Conventionally, some diesel engines have a selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flows, said catalyst having a feature of selectively reacting NOx (nitrogen oxides) with a reducing agent even in the presence of oxygen. A required amount of reducing agent is added upstream of the reduction catalyst and is reductively reacted on the catalyst with NOx in the exhaust gas to thereby reduce a concentration of NOx discharged.
Meanwhile, effectiveness of ammonia (NH3) used as a reducing agent for reduction and purification of NOx is well known in a field of industrial flue gas denitration in a plant or the like. However, in a field of automobile where safety is hard to assure as to running with ammonia itself being loaded, it has been proposed to use nontoxic urea water as reducing agent.
Specifically, the urea water added into the exhaust gas upstream of the selective reduction catalyst is thermally decomposed into ammonia and carbon dioxide gas according to the following formula to satisfactorily depurate NO in the exhaust gas through reduction on the catalyst by the ammonia.(NH2)2CO+H2O→2NH3+CO2  [Chemical Formula 1]
FIG. 1 is an example of conventional art for addition of urea water into exhaust gas upstream of a selective reduction catalyst. In the example illustrated, an exhaust pipe 1 has an outer periphery formed with a bulge portion 2 protruding obliquely upstream. Mounted on an upstream side of the bulge portion 2 is an injector 3 which is directed obliquely downstream to approach to an inside of the exhaust pipe 1 so as to add urea water 5 while protecting the injector 3 from being directly exposed to the flow of the hot exhaust gas 4.
Arranged in the exhaust pipe 1 downstream of the added position of the urea water 5 by the injector 3 is a lattice board 6 on which hit is urea water 5 sprayed from the injector 3 for dispersion and facilitated vaporization of the urea water 5. As enlargedly shown in FIG. 2, the lattice board 6 is axially provided with a number of flow passages 6a. The flow passages 6a are provided, on their discharge sides, with alternately reversely directed fins 6b so that flows of the exhaust gas 4 discharged from the flow passages 6a are sorted alternately in reverse directions, which facilitates hitting and dispersion of the urea water 5 on the fins 6b and into the exhaust gas 4.
With respect to this kind of mixing structure, there exists, for example, the following Patent Literature 1 as conventional art document.